Research has been conducted for many years on IP multicast which distributes an IP packet sent by a certain sender to a group (IP multicast address) to all recipients belonging to the multicast group. As one sort of it, reliable multicast which performs confirmation of reception has also been studied. In recent years, a large amount of research has been conducted on overlay multicast particularly because of its easiness in being put into widespread use. For example, representative researches on them are introduced in Non-patent Literature (NPL) 1.